Electromagnetic waves generated by internal components of various electronic instruments such as mobile communication terminals, notebook computers, business machines and medical instruments allegedly may cause a variety of disorders such as headache, visual acuity reduction, leukemia, brain tumors, circulation system abnormalities and sterility. In addition, with the increased degree of manufacturing integration and trends towards miniaturization and weight reduction in electronic products, electromagnetic noise generated by product elements may cause product malfunction. To address these problems, regulations have been introduced to limit the electromagnetic (EM) emissions and radio interference caused by some electronic products, including computers, wireless phones, medical instruments and multimedia players. Accordingly, methods of shielding electromagnetic emissions generated by certain electronic products have come to play an important role in product design and industrial manufacturing processes.
Conventional shielding methods for blocking electromagnetic waves include plating, vacuum evaporation, spray-coating and similar methods. Electromagnetic wave shielding methods utilizing plating have been used extensively, but may suffer from relatively high production costs, complex production processes and environmental contamination. In addition, shielding of electromagnetic waves using vacuum evaporation can be relatively expensive and suffer from long term reliability problems. In contrast, electromagnetic wave shielding techniques employing spray-coating of metal powder may support relatively easy application methods and result in relatively little environmental contamination.
Methods of spray-coating typically include spraying a coating solution onto a substrate. The coating solution typically contains a mixture of an adhesive resin and conductive metal. To be useful in most manufacturing processes, the conductive coating should have excellent abrasion resistance to withstand scratching caused by back-end manufacturing steps. The conductive coating should also have good adhesion characteristics to prevent peeling of the conductive coating and contamination and failure of surrounding electronic components.
In a conventional spray-coating method, a metal powder, such as a flake-like silver powder, nickel powder or aluminum powder, may be used to provide electrical conductivity within an otherwise relatively highly resistive paint composition. Unfortunately, the metal powder within a paint composition may settle and form a sediment within the paint composition. The formation of a sediment may cause layer separation that is difficult to correct with conventional mixing techniques such as mechanical stirring. To inhibit sedimentation, the paint composition should have sufficiently high viscosity. An example of a conventional spray-coating composition is disclosed in U.S. Pat. No. 6,645,613 to Ricca. In particular, Ricca discloses a coating composition containing an aqueous polyurethane dispersion having an alkyl chain structure.
Some polyurethanes may be used as adhesive resins within spray coatings if they possess sufficient flexibility, rebound resilience and abrasion resistance and provide sufficiently strong adhesiveness. Conventional polyurethanes have frequently been prepared and used as oily preparations and may utilize organic solvents such as methyl ethyl ketone (MEK), dimethylformamide and toluene in order to impart sufficient viscosity to the preparation. Unfortunately, the use of organic solvents may contribute to environmental pollution and may cause health problems in persons exposed to the preparation. The use of organic solvents may also pose a substantial fire risk. To address these problems, water-dispersible polyurethane resins have been developed. Some of the water-dispersible polyurethane resins are manufactured using spontaneous emulsification methods and have a chemical structure that includes ionic groups on a polymer backbone. Methods for imparting ionic groups such as cationic ionomers to a main chain of a polyurethane are disclosed in U.S. Pat. Nos. 4,016,123, 4,190,567 and 4,277,383, and in Japanese Patent Laid-Open Application No. Hei 5-320331. Additional methods for imparting ionic groups such as anionic ionomers to a main chain of a polyurethane are disclosed in U.S. Pat. Nos. 4,016,122 and 4,914,148 and in Japanese Patent Laid-Open Application No. Hei 5-39340. U.S. Pat. No. 4,794,147 discloses a method of imparting a non-ionic compound to a main chain of a polyurethane.
Water-dispersible polyurethane resins may be broadly classified as polyether, polyester and acrylate-based dispersions, depending on the type of polyols utilized in polymerization. Among these, polyester-based polyols, which typically exhibit strong adhesiveness to various materials and excellent mechanical and physical properties, such as abrasion resistance, are widely used. A polyester-based polyol may be synthesized by reacting a polyfunctional acid with a polyhydric alcohol. Polyfunctional acids utilized in the synthesis of polyester-based polyols include aliphatic acids such as adipic acid, and aromatic acids such as benzoic acid, isophthalic acid, terephthalic acid, phthalic anhydride and anhydride.